Filter Choices from GAL?

So I'm trying to filter a combobox to display a list of people that fit a specific filter (in this instance, people whose "department" = a specific text string).

The combobox's "Items" property currently has:

Choices([@Coaching_1].OData__x0061_hf6)

which looks like it pulls a list of people from the company's global address list.

How do I edit the Items property to only display people from a specific department rather than the whole company? I've done some searching and used various methods of using the Filter property to try and narrow down the combobox, but nothing appears to be working.

An example of one use of filter I've tried that I think is close but yields no results is

Re: Filter Choices from GAL?

Thanks for the suggestion. I did read that post up and down multiple times and attempted to apply what I could understand from it to my formula with no success. At some point I did get it to at least not display formula errors, but when trying to use the combobox and type in names, the combobox yields no results at all.